1. Field of the Invention
The invention relates to correction of a focusing error signal in an optical storage system, more particularly to a method and device for canceling a land-groove offset component of a focusing error signal in an optical storage system.
2. Description of the Related Art
Referring to FIG. 1, a conventional optical storage system 1 is shown to include an optical pickup 11 for reading data from or recording data into an optical disc 12, a spindle motor 13 for driving rotation of the optical disc 12, a sled motor 14 for driving movement of the optical pickup 11, a focus coil motor 15 associated operably with the optical pickup 11, a preamplifier 10, a power driver 16, an analog-to-digital (A/D) converter 17, and a digital signal processor (DSP) 18. The DSP 18 is operable so as to control the focus coil motor 15 via focusing servo control information.
The optical pickup 11 includes a light source, an object lens, and a photo detector. When the optical storage system 1 reads data, a main beam from the light source is focused on the optical disc 12 through the object lens. The light beam reflected by the optical disc 12 is detected by the photo detector through the object lens. With the change in focused position, for instance, the groove 121, the land 122, or different data states 123 in the groove 121, the amount of reflected light varies accordingly. The photo detector detects the reflected light, and converts the same into electrical signals having corresponding voltage values (or electrical current values). The electrical signals from the photo detector are processed by the preamplifier 10 and the A/D converter 17 prior to receipt thereof by the DSP 18.
Therefore, data recorded in the optical disc 12 and associated servo control information will be converted into electrical signals through the optical pickup 11 that are subsequently provided to the preamplifier 10. The output signal of the preamplifier 10 includes a radio frequency signal (RF) that is an indication of the data read from the optical disc 12, and a focusing error signal (FE) that is an indication of the focusing error. The radio frequency signal (RF) will be decoded by the optical storage system 1 to determine the content of the data read from the optical disc 12. On the other hand, the focusing error signal (FE) will be processed by the DSP 18 to generate a focusing control signal (FOO) In response to the focusing control signal (FOO), the power driver 16 controls operation of the focus coil motor 15 to adjust the position of the object lens so as to correct the focused light spot on the optical disc 12. As shown in FIG. 2, the disc reflected signal may be divided into four regions (A), (B), (C) and (D), corresponding to four regions (A), (B), (C) and (D) of the photo detector of the optical pickup 11. The preamplifier 10 generates the focusing error signal (FE) from the reflected signals. One example of the FE signal is FE=(A+C)−(B+D). Therefore, the DSP 18 is operable to determine whether or not the light beam is accurately focused based on the numerical value of the focusing error signal (FE) and, when the numerical value of the focusing error signal (FE) is non-zero, provides the focusing control signal (FOO) to the power driver 16 such that the power driver 16 controls operation of the focus coil motor 15 to adjust the position of the object lens accordingly. The aforesaid feedback control operation performed by the optical storage system 1 through the focusing error signal (FOO) is commonly known as focusing servo control. In addition, the optical storage system 1 is also operable to generate other forms of servo control, such as track locking servo control, track seeking servo control, etc. Thus, the preamplifier 10 further generates a tracking error signal (TE) and a radio frequency ripple signal (Rfrp) or a wobble ripple signal so as to enable the DSP 18 to perform the other types of servo control.
It is noted that current recordable optical discs, such as CD-R, CD-RW, DVD-RAM, DVD±R, DVD±RW, etc., are designed to include wobble signals (sinusoidal wobbling of the land) that carry timing information. As a matter of fact, the reflected amounts of light from the groove 121 and the land 122 vary inherently. Therefore, when a track is being crossed by the focused spot 1, the amounts of reflection detected in the regions (A), (B), (C), (D) of the photo detector will fluctuate. Under an ideal condition, the line that separates regions (B) and (C) from regions (A) and (D) of the photo detector should be parallel to the boundary of the groove 121 and the land 122, as best shown in FIG. 3, such that fluctuations in the reflected amounts of light during track crossing do not influence generation of the focusing error signal (FE) because regions (A) and (D) always have the same fluctuations, and their effect is minimized in the formation of the focusing error signal (FE).
However, in practice, due to manufacturing factors, such as PDIC assembly errors, wobbling of the land-groove boundary, etc., the line that separates regions (B) and (C) from regions (A) and (D) of the photo detector is seldom parallel to the boundary of the groove 121 and the land 122, as shown in FIG. 4. As a result, during track crossing, the focusing error signal (FE) is always superimposed by a land-groove offset component. The presence of the land-groove offset component is evident in case of a track seeking operation, where the land 122 and the grooves 121 are continuously crossed. Therefore, during the track crossing operation, the focusing error signal (FE) fluctuates such that the DSP 18 normally makes an incorrect conclusion as to the presence of improper focusing of the optical pickup 11. As a result, the DSP 18 varies the focusing control signal (FOO) in order to compensate for the superimposed focusing error, i.e., the land-groove offset, as shown in FIG. 5. However, since there is actually no problem with the focusing position, the DSP 18 merely generates a series of meaningless control signals for driving the focus coil motor 15 to perform unnecessary adjustments, which not only does not yield any beneficial effect, but also results in waste of energy and harmful heat.
For further details, referring to FIGS. 5A to 5D, when the optical storage system 1 performs track crossing, an increase in the amplitude of the tracking error signal (TE) (see FIG. 5A) and in the amplitude of the focusing error signal (FE) (as shown in FIG. 5B) due to the effect of the land-groove offset component results in an increase in the amplitude of the focusing control signal (FOO) (see FIG. 5D) from the DSP 18. The increase in the amplitude of the focusing control signal (FOO) can result in saturation and subsequent lock-off condition of the servo loop, which affects the stability of focusing control during track seeking and limits the allowable frequency bandwidth for focusing control. On the other hand, for long periods of track seeking operations, since large land-groove offset components are present in the focusing error signal (FE) due to continuous track crossings, the power driver 16 outputs control currents to the focus coil motor 15 continuously for relatively long periods of time, which heats up the focus coil motor 15, thereby shortening the service life and affecting the reliability of the same.
Therefore, to solve the aforesaid problem, many techniques have been proposed heretofore to lower the output for focusing control during track crossing in an optical storage system 1. For example, in U.S. Pat. No. 4,747,089, there is disclosed a method and apparatus for canceling a land-groove offset component of a focusing error signal in an optical storage system by: detecting the focusing error signal synchronously with a tracking error signal to produce a first periodic function signal; detecting the focusing error signal synchronously with an RF signal to produce a second periodic function signal; converting the tracking error signal into a sine wave signal; converting the RF signal into a cosine wave signal; multiplying the first periodic function signal by the cosine wave signal to produce a first component signal; multiplying the second periodic function signal by the sine wave signal to produce a second component signal; and adding the first and second component signals to produce a disturbance signal that is to be subtracted from the focusing error signal to cancel the land-groove offset component therefrom. The scheme proposed in U.S. Pat. No. 4,747,089 is disadvantageous in that it involves a complex architecture to perform complex calculations and is thus costly to implement.
In U.S. Pat. No. 5,199,011, there is disclosed another apparatus for canceling a land-groove offset component of a focusing error signal in an optical storage system. In the proposed apparatus, a filtered calibration focusing error signal is generated during a calibration phase of an optical disc drive, and a correction factor signal is generated from the filtered calibration focusing error signal as a function of a tracking error signal. The correction factor signal is then used to modify the focusing error signal to cancel the land-groove offset component therefrom. The apparatus proposed in U.S. Pat. No. 5,199,011 is disadvantageous in that generation of the correction factor signal requires a large amount of memory space and sampling operations that are relatively difficult to implement.